A pipeline laid on the bottom of the sea allows a high-pressure fluid to flow therein. The pipeline is further subjected to repeated distortion caused by waves and subjected to a seawater pressure. Therefore, a steel pipe used for the pipeline on the bottom of the sea is required to have high strength and high toughness.
In recent years, oil wells and gas wells in a sour environment, especially in the deep sea or in cold climates, severer than the conventional environment is under development. The undersea pipeline laid in such a severe sour environment is required to have strength (pressure resistance) and toughness higher than the conventional ones.
For the undersea pipeline, which is required to have such properties, a seamless steel pipe is more suitable than a welded steel pipe. This is because the welded steel pipe has a weld zone (seam portion) along the longitudinal direction. The weld zone has a toughness lower than that of a base material. Therefore, the seamless steel pipe is suitable for the undersea pipeline.
A thicker wall of the seamless steel pipe provides high strength. However, the increase in wall thickness easily causes a brittle fracture and decreases the toughness. Therefore, the thick-wall seamless steel pipe is required to have excellent toughness. In order to improve the strength and toughness for the thick-wall seamless steel pipe, it is only necessary to increase the content of alloying elements such as carbon to enhance the hardenability. However, in the case where the seamless steel pipes having improved hardenability are joined to each other by circumferential welding, the heat affected zone is liable to harden, and the toughness of the weld zone formed by circumferential welding decreases. For the thick-wall seamless steel pipe used for the undersea pipeline, the base material and weld zone thereof are required to have excellent toughness.
JP2000-104117A (Patent Document 1), JP2000-169913A (Patent Document 2), JP2004-124158A (Patent Document 3), and JP9-235617A (Patent Document 4) propose methods for manufacturing a seamless steel pipe for line pipe, for improving the toughness thereof.